Holographic recording system for large objects

ABSTRACT

A holographic arrangement for recording a high quality hologram of an object having a length many times the coherence length of the coherent light waves employed in recording the hologram.

.1 III UV Lita!" Gorog et al.

[ HOLOGRAPHIC RECORDING SYSTEM FOR LARGE OBJECTS lnventors: lstvanGorog, Princeton; Fred William Spong, Lawrenceville, both of NJ.

OTHER PUBLICATIONS 45 Bell Laboratories Record" 238-239 (July-Aug. 1967)Bobrinev et al., 13 Radio Engineering and Electronic Physics" 1814, I815(ll/I968) [73] Assignee: RCA Corporation, New York, N.Y.

Primary ExaminerRonald L. Wibert lzzl Flled: Assistant ExaminerRonald J.Stern v [2]] APPL 212,503 AttorneyEdward J. Norton and George J.Seligsohn 52 u.s. Cl. 350/15 [571 ABSTRACT [51 Int. Cl. G02b 27/00 Aholo ra hic arrangement for recording a high quality [58] Field ofSearch BSD/5.5 h lo ram of an object having a length many times thecoherence length of the coherent light waves employed [56] ReferencesCited in recording the hologram.

UNITED STATES PATENTS 3 Claims, 2 Drawing Figures 3,644,047 2/l972 Brownet al 356/109 SINGLE FREQUENCY MODE LASER (CAVITY LENGTH= L) i MIRROR lI02 I .IOO I 108 BEAM 08 SPLITTER CONCAVE I06 mRROR MIRROR E 4 HO I I22-I 5 Q I20 l22-3 I24 OBJECT ILB REFERENCE ILLUMINATING BEAM MIRROR ||6HOLOGRAM A/ RECORDING l26-3 MEDIUM OBJECT BEAM DIFFUSELY REFLECTINGLARGE OBJECT SPACE Pmmmmsm 3.740.111

SINGLE FREQUENCY MODE LASER (CAVITY LENGTH=L) MIRROR E I02,- I

OBJECT DIFFUSELY REFLECTING A BEAM LARGE OBJECT SPACE VISIBILITY 0 2L/34L/3 2L 8L/3 PATH DIFFERENCE HOLOGRAPIIIC RECORDING SYSTEM FOR LARGEOBJECTS This invention relates to a holographic recording system and,more particularly, to such a system capable of recording a hologram ofan object which is large relative to the coherence length of coherentlight waves employed for recording the hologram.

As is known, coherence length is defined as the velocity of lightdivided by the frequency bandwidth of the coherent light waves employedin recording a hologram. Therefore, unless the bandwidth is zero, thecoherence length is finite. Only a single frequency laser which isstabilized against an absolute frequency standard provides a single,nearly pure time coherent sinusoidal wave having an effective bandwidthapproaching zero. If a single frequency laser is not stabilized againstan absolute frequency standard, then environmental changes andfluctuations will change the laser frequency over the hologram recordingexposure interval.

It has been found experimentally that, during a typical hologramexposure time of ten seconds, the laser frequency of a single frequencylaser, such as an argon laser utilizing an intra-cavity three-mirrorinterferometer, may change as much as 500 MHz. An effective lightbandwidth of 500 MHz corresponds to a coherence length of 0.6 meter.This implies that ifa standard holographic arrangement is used, objectslarger than 0.6 meter cannot be holographed with a 500 MHz bandwidthlaser.

The present invention is directed to a novel holographic arrangementwhich makes it possible to record a hologram of all objects situatedwithin a given object space having a length which is many times thecoherence length of the coherent light waves employed in recording thehologram.

This and other features and advantages of the present invention willbecome more apparent from the following detailed description, takentogether with the accompanying drawing, in which:

FIG. 1 is a diagram illustrating an embodiment ofthe holographicarrangement employed in the present invention, and

FIG. 2 is a graph helpful in explaining a principle of the presentinvention.

Referring now to FIG. 1, there is shown laser 100, which has a cavitylength of L and which operates at least approximately in a singlefrequency mode. The term approximately, as just used, applies to asingle frequency laser which is not stabilized against an absolutefrequency standard and which may be scanned or otherwise modulated, sothat the output light waves therefrom have a limited coherence length.

For the purposes of clarity in the drawing, various light beams depictedtherein are represented solely by their respective center lines. Inparticular, output beam 102 from laser 100, after being reflected fromfirst plane mirror'104, is incident on beam splitter 106. Beam splitter106, which may consist of a partially reflecting mirror, dividesincident output beam 102 of coherent light waves into first component108, reflected from beam splitter 106, and second component 110,transmitted through beam splitter 106.

Second component 110, after reflection from second plane mirror 112,forms reference beam 114, which is directly incident on a given area ofhologram recording medium 116.

First component beam of coherent wave energy, after reflection fromthird plane mirror 118, is incident on concave mirror 120. In responsethereto, concave mirror reflects light waves, which first converge,crossover and then diverge, to provide in the far field divergent beam122 of the first component'of the coherent light waves. Divergent beam122 can be considered to be composed of a plurality of light rays, suchas light ray 122-1, 122-2 and 122-3, which are incident on objectilluminating mirror 124 at somewhat different angles of incidence withrespect to each other. Each incident light ray illuminating objectilluminating mirror 124 will give rise to a corresponding reflected beam126 composed of rays, such as light rays 126-1, 126-2 and 126-3, of thefirst component of the coherent light waves.

As shown, the reflected light rays of beam 126, such as light rays126-1, 126-2 and 126-3, are directed into a given large object space 126in which may be located either a single large object or a plurality ofdistributed smaller objects which, in either case, diffusely reflectincident light. Therefore, each point of an object within object space126, in response to being illuminated by a ray of reflected light beam126 from object illuminating mirror 124, such as rays 126-1, 126-2 and126-3, gives rise to diffusely-reflected first-component light of thecoherent light waves. Some of this diffusely reflected light, such as128-1, 128-2 and 128-3, will form an object beam 128 composed of lightfrom the first component of the coherent wave energy which is directedtowards and is incident on the same given area of hologram recordingmedium as the reference beam 114 composed of the second component of thecoherent light waves. A hologram is formed on the given area of hologramrecording medium 116 by the interference between incident object beam128 and incident reference beam 114, which, as shown, are angularlydisplaced with respect to each other.

Before discussing the operation of the holographic arrangement shown inFIG. 1, certain known principles of light waves which are applicable tothe present invention will be discussed. First, if two light beams thatare derived from the same source interfere, and the source spectrum isperiodic, then the interferencefringe visibility is a periodic functionof the path length difference between the two beams. Second, thefundamental transverse mode laser has the property that, to a highdegree of accuracy, the laser frequency is an integer multiple of C/2L,where C is the speed oflight and L is the cavity length. Therefore, iffluctuations force the frequency of a single frequency laser to change,then it will also change in a quantized manner by multiples ofapproximately C/2 L.

Merely as an illustrative example, consider the interference between twoseparate, equal-intensity beams derived from a single frequency laser.Assume in this example that during the exposure interval of therecording of the interference fringe pattern, the laser frequency ischanged in such a manner that one-third of the exposure is obtained witheach of three uniformly spaced frequency components. Then, one can showthat the relative fringe visibility as a function of the path differencebetween the two beams is as shown in the graph of FIG. 2. As can be seenfrom FIG. 2, the fringe visibility is perfect as long as the pathdifference is an even integer multiple of the laser cavity length.Furthermore, it can be shown that this rule holds for an arbitrarynumber of equally spaced frequencies, although the number of lobesincrease while the width of each lobe decrease.

Since holography is based on the two beam interference phenomena, theabove concept shows that high quality holograms can be made withmulti-frequency exposures provided that the path difference betweenobject and reference beams is either equal to an even integral multipleof the laser cavity length or varies therefrom by an amount no greaterthan the coherence length of the coherent light waves emitted by thelaser.

Referring again to FIG. 1, the relative locations of hologram recordingmedium 116, object illuminating mirror 124 and diffusely reflectinglarge object space 126 are selected with respect to each other so thatthe sum of any one of the rays of reflected beam 126, such as rays126-1, 126-2, and 126-3, incident on a point of a diffusely reflectingobject, and corresponding rays of object beam 128, such as rays 128-1,128-2 and 128-3, differ from a given path length by no more than plus orminus the coherence length of the coherent light waves from laser 100.Further, by properly selecting the location of plane mirror 112, thepath difference between the overall length of the second component ofthe coherent light waves forming reference beam 114 incident on hologramrecording medium 116 and the overall length of the first component ofcoherent light waves forming object beam 128 incident on hologramrecording medium 116 can be made to vary from a given even integralnumber of cavity lengths by no more than the coherence length of thecoherent light wave emitted by laser 100. In this manner, a hologram maybe recorded of all the object situated within a large object spacehaving a length which is significantly longer than the coherence lengthof the coherent light waves employed in recording the hologram.

By way of example, it has been found that a high quality hologram of anobject having a length of about 6 feet can be recorded with coherentlight waves having a bandwidth of about 6 gigahertz. Since the coherencelength of such coherent light is only about 2 inches, it can be seenthat objects substantially greater than thirty times the coherencelength of the coherent light waves employed can be accommodated by theholographic arrangement of the present invention.

What is claimed is:

1. A holographic recording system for recording a hologram of diffuselyreflecting objects located in a given object space having a first givenlength employing coherent light waves having a coherence lengthsignificantly smaller than said first given length, said systemcomprising:

a. means including at least an approximately singlefrequency mode laserhaving an optical resonant cavity of a second given length for producingfirst and second mutually coherent components of coherent light waveseach having said coherence length,

b. a hologram recording medium having a first selected location chosenwith respect to said given object space,

c. object illuminating means comprising a single object-illuminatingplane mirror and a single concave mirror having respective second andthird locations chosen with respect to both said given object space andsaid medium so that illumination of said concave mirror with said firstcomponent of said herent light waves reflects a single divergent beam ofcoherent light waves incident on said plane mirror having rays at thoseangles that result in the illumination of each point of said givenobject space with an illuminating ray of a single divergent beam oflight waves reflected from said plane mirror, thereby to cause theillumination of said hologram recording medium with an object beamcomprising reflected rays of said coherent light waves diffuselyreflected from each illuminated point of any object within said givenobject space, said first and third selected locations being such thatthe sum of the lengths of any pair of illuminating and reflected rays ofany one illuminated point of an object differs from a predeterminedlength by no more than said coherence length, and

d. means for illuminating said concave mirror with said incident beam offirst component of said coherent light waves to derive the light whichilluminates said hologram recording medium as said object beam andsimultaneously directly illuminating said hologram recording medium witha reference beam of said second component of said coherent light waveswhich is angularly displaced from said object beam, the difference inthe overall length of the path of light derived from saidfirst-component of said coherent light waves which illuminates saidhologram recording medium as said object beam and the overall length ofthe path of light which illuminates said hologram recording mediumdirectly as said reference beam being substantially equal to an evenintegral multiple of said second given length.

2. The system defined in claim 1, wherein said first given length is atleast thirty times said coherence length.

3. A system for recording on a hologram medium an object having a firstgiven length and located in a given object space employing coherentlight waves having a coherence length significantly smaller than saidfirst given length, said hologram medium having a first selectedlocation chosen with respect to said given object space, said systemcomprising:

a. means for producing from an approximately singlefrequency mode laserhaving an optical resonant cavity of a second given length first andsecond mutually coherent components of coherent light waves each havingsaid coherence length,

b. object illuminating means comprising a single object-illuminatingplane mirror and a single concave mirror having respective second andthird locations chosen with respect to both said given object space andsaid medium so that illumination of said concave mirror with said firstcomponent of said coherent light waves reflects a single divergent beamof coherent light waves incident on said plane mirror having rays atthose angles that result in the illumination of each point of said givenobject space with an illuminating ray of a single divergent beam oflight waves reflected from said plane mirror, thereby to cause theillumination of said medium with an object beam comprising reflectedrays of said coherent light waves diffusely reflected from eachilluminated point of any object within said given object space, saidfirst and third selected locations being such that the sum of thelengths of any pair of illuminating and reflected rays of any oneilluminated point of an object differs from a predetermined length by nomore than said coherence length, and

. means for illuminating said concave mirror with said incident beam ofsaid first component of said coherent light waves to derive the lightwhich illuminates said medium as said object beam and simultaneouslydirectly illuminating said medium with a reference beam of said secondcomponent of said coherent light waves which is angularly dissecondgiven length.

t t t i t

1. A holographic recording system for recording a hologram of diffuselyreflecting objects located in a given object space having a first givenlength employing coherent light waves having a coherence lengthsignificantly smaller than said first given length, said systemcomprising: a. means including at least an approximatelysingle-frequency mode laser having an optical resonant cavity of asecond given length for producing first and second mutually coherentcomponents of coherent light waves each having said coherence length, b.a hologram recording medium having a first selected location chosen withrespect to said given object space, c. object illuminating meanscomprising a single objectilluminating plane mirror and a single concavemirror having respective second and third locations chosen with respectto both said given object space and said medium so that illumination ofsaid concave mirror with said first component of said coherent lightwaves reflects a single divergent beam of coherent light waves incidenton said plane mirror having rays at those angles that result in theillumination of each point of said given object space with anilluminating ray of a single divergent beam of light waves reflectedfrom said plane mirror, thereby to cause the illumination of saidhologram recording medium with an object beam comprising reflected raysof said coherent light waves diffusely reflected from each illuminatedpoint of any object within said given object space, said first and thirdselected locations being such that the sum of the lengths of any pair ofilluminating and reflected rays of any one illuminated point of anobject differs from a predetermined length by no more than saidcoherence length, and d. means for illuminating said concave mirror withsaid incident beam of first component of said coherent light waves toderive the light which illuminates said hologram recording medium assaid object beam and simultaneouslY directly illuminating said hologramrecording medium with a reference beam of said second component of saidcoherent light waves which is angularly displaced from said object beam,the difference in the overall length of the path of light derived fromsaid first-component of said coherent light waves which illuminates saidhologram recording medium as said object beam and the overall length ofthe path of light which illuminates said hologram recording mediumdirectly as said reference beam being substantially equal to an evenintegral multiple of said second given length.
 2. The system defined inclaim 1, wherein said first given length is at least thirty times saidcoherence length.
 3. A system for recording on a hologram medium anobject having a first given length and located in a given object spaceemploying coherent light waves having a coherence length significantlysmaller than said first given length, said hologram medium having afirst selected location chosen with respect to said given object space,said system comprising: a. means for producing from an approximatelysingle-frequency mode laser having an optical resonant cavity of asecond given length first and second mutually coherent components ofcoherent light waves each having said coherence length, b. objectilluminating means comprising a single object-illuminating plane mirrorand a single concave mirror having respective second and third locationschosen with respect to both said given object space and said medium sothat illumination of said concave mirror with said first component ofsaid coherent light waves reflects a single divergent beam of coherentlight waves incident on said plane mirror having rays at those anglesthat result in the illumination of each point of said given object spacewith an illuminating ray of a single divergent beam of light wavesreflected from said plane mirror, thereby to cause the illumination ofsaid medium with an object beam comprising reflected rays of saidcoherent light waves diffusely reflected from each illuminated point ofany object within said given object space, said first and third selectedlocations being such that the sum of the lengths of any pair ofilluminating and reflected rays of any one illuminated point of anobject differs from a predetermined length by no more than saidcoherence length, and c. means for illuminating said concave mirror withsaid incident beam of said first component of said coherent light wavesto derive the light which illuminates said medium as said object beamand simultaneously directly illuminating said medium with a referencebeam of said second component of said coherent light waves which isangularly displaced from said object beam, the difference in the overalllength of the path of light derived from said first-component of saidcoherent light waves which illuminates said hologram recording medium assaid object beam and the overall length of the path of light whichilluminates said hologram recording medium directly as said referencebeam being substantially equal to an even integral multiple of saidsecond given length.